1. Field of the Invention
The present invention pertains to a method of double-sided etching, and more specifically, to a method of forming a micro spindle by double-sided etching.
2. Description of the Prior Art
Micro electromechanical system (MEMS) technology is an emerging technology which highly integrates electronics and mechanics, and has been broadly adopted to fabricate various devices having electro and mechanical characteristics, such as micro sensors, micro actuators, micro motors, photosensitive switches, etc. MEMS devices generally have mechanical structures more complicated than semiconductor devices, and thus cannot be directly fabricated by standard semiconductor processes. Take the micro spindle, one of the most common structures in MEMS devices, for example, accurate shape and smooth surface condition are strictly required for ensuring reliability and stress-bearing capability.
Please refer to FIG. 1 to FIG. 3. FIG. 1 is a schematic diagram of a micro spindle 10; FIG. 2 and FIG. 3 are schematic diagrams illustrating a conventional method of forming a micro spindle. As shown in FIG. 1, the micro spindle is a suspension structure capable of being driven by a voltage, light beams, or a magnetic field, and thereby rotates in the direction as the arrow indicates shown in FIG. 1. Therefore, the micro spindle 10 must have an accurate shape, a smooth surface, and a uniform axis so as to guarantee reliability and stress-bearing capability. The conventional method of fabricating the spindle 10 is described below.
As shown in FIG. 2, a wafer 20 is provided. Then, an etching stop layer 22 and a photo resist pattern 24 are respectively formed on the bottom surface and the top surface of the wafer 20. As shown in FIG. 3, an etching process is followed to etch through the wafer 20 not covered by the photo resist pattern 24 until the etching stop layer 22.
According to the conventional method, however, equilibrium of the etching process and the thickness uniformity of the wafer 20 are not taken into consideration. Thus, the yield of the etching process cannot be well controlled because etching rates in different regions of the wafer 20 are not equal. For example, on the occasion of etching through the wafer 20, the overall area being etched varies dramatically, and leads to unexpected changes during the etching process. In addition, when the etching process is performed down to the etching stop layer 22, side etching effect tends to occur and therefore results in an undercut 26 as shown in FIG. 3. As mentioned earlier, if the shape accuracy of the micro spindle degrades, reliability of the micro spindle will be seriously affected.